Septic tanks which employ a membrane separation system are known as equipment for purifying septic tank sludge, household wastewater sludge, night soil or the like.
As disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. JP-A-3-213128) or the like, such septic tanks in general have a structure in which raw water is filtered by a separation membrane immersed in the raw water stored in a treatment tank. Meanwhile, in such immersion-type membrane separation apparatus, as filtration continues on, impurities such as solids will adhere to the separation membrane, causing deterioration of filtration ability. Therefore, it is necessary to clean the separation membrane and remove the impurities adhering thereto.
Conventionally, as a method for cleaning the separation membrane, there have been proposed a method utilizing an ultrasonic wave as disclosed in Patent Document 1 or the like. Specifically, in this method, an ultrasonic wave cleaning apparatus exerts an ultrasonic oscillation on the surface of the filtration membrane so that oscillation of the membrane surface as well as interference between the oscillating membrane surface and the raw water will clean and remove matters adhering to the membrane surface such as the gel layer.
However, such ultrasonic wave cleaning apparatus have not yet been commercially available due to a significant problem for practical use. The problem is an issue of durability of the filtration membrane. That is, it has been found that if the ultrasonic oscillation is exerted directly on the surface of the filtration membrane, the separation membrane may be damaged early, resulting in problems of dispersing into the raw water in pieces.
Additionally, Patent Document 2 (Japanese Unexamined Patent Publication No. JP-A-11-319517) proposes a method of cleaning a separation membrane in which an ultrasonic transducer emits an ultrasonic wave so as to generate cavitation bubbles within the raw water, and an impact force created during disappearance of the cavitation bubbles is exerted on the separation membrane. However, in such a cleaning method, the entire membrane separation apparatus will be subjected to the impact force created during disappearance of the cavitation bubbles. This may pose a problem of durability of not only the separation membrane but also the membrane separation apparatus itself.